The big news seemed to be Senator Warren's appointment to the Banking Committee, but I've compiled a list of all the committee churn for the Democrats. HELP is Health, Education, Labour, and Pensions. HSGAC is Homeland Security and Governmental Affairs. For returning members in the second section in parenthesis are the committee(s) they gave up.
Baldwin - HELP, HSGAC, Aging, Budget
Donnelly - Agriculture, Armed Services, Aging
Murphy - Foreign Relations, HELP, Joint Economic
Hirono - Energy, Judiciary, Veterans' Affairs
Heinrich - Energy, Intelligence, Joint Economic
King - Armed Services, Intelligence, Budget, Rules
Kaine - Armed Services, Foreign Relations, Budget
Warren - Banking, HELP, Aging
Heitkamp - Agriculture, Banking, HSGAC, Small Business, Indian Affairs
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Sherrod Brown - Finance (Appropriations)
T. Udall - Appropriations (Commerce)
Shaheen - Appropriations (Energy)
Merkley - Appropriations (HELP)
Begich - Indian Affairs (Budget)
Bennet - Finance (Banking, Aging)
Manchin - Banking (none)
Blumenthal - Commerce, Veterans' Affairs (HELP)
Some thoughts. Warren wasn't the only incoming freshman appointed to Banking, and she will actually have more seniority than Heitkamp by virtue of being from a larger state. On the other hand Manchin got a seat on Banking as a returning senator so it would seem somewhat desireable. Coons was the only senator from the 2010 class not to change any of his assignments while Blumenthal made several changes. Sherrod Brown[his first name is spelled out to avoid confusion with Scott Brown[buh bye]] leaves Appropriations for Finance having just won reelection, while 3 senators up for their first reelection in 2014 are added to Appropriations. This may just be a coincidence as Appropriations is always going to be attractive, but the cynic in me thinks this is intentional to help their reelection efforts. Angus King seemed to get rather plum assignments, possibly as consideration for his decision to caucus with the Dems, but that could be my imagination.